Even if Ein isn't written officially in the DOA storyline as of late, the story is still open-ended enough for the possibility for him to be so. Remember, Hayate was caught in the M.I.S.T. lab for diagnosis, which could lead to Ein's canon return to the official story come DOA6.
I dunno about you guys, but the idea of Ein being an evil badass working for M.I.S.T. is very interesting. This opens up so many possibilities for character development in areas that needed it, such as Hitomi getting over her over-reliance on Ein for emotional support (and effectively bringing back the personality she once had in DOA3) and getting more deeply involved in the inner workings of M.I.S.T. rather than just playing second fiddle to the ambiguously lesbian duo that is her and Leifang in random antics that make them too similar to Asuka and Lili (not that I mind it personally, but sometimes you gotta give it a rest).